With the increasing need for high-capacity, low-power storage used for multimedia applications, mobile communications and etc., semiconductor device market shared by non-volatile memory, especially flash memory, is becoming larger and larger, and becoming a very important type of memory. The primary characteristic of the nonvolatile memory is that it can preserve the stored information for a long period of time without power, having both the characteristic of read-only memory and very high access speed.
The nonvolatile memory on the market today is dominated by flash memory, but flash devices have disadvantages such as overhigh operating voltages, low speed of operating, not good enough endurance, and short retention time due to the too thin tunnel oxide layer during the device shrinking. Ideal nonvolatile memory should have the conditions of low operating voltage, simple structure, non-destructive reading, fast operation, long retention time, good endurance and excellent scalability. A number of new materials and devices have been studied to try to achieve the above objectives, in which a significant portion of the new memory devices use the change of resistance value as a way of memory, including resistive switching memory and resistance switching memory adopting solid electrolyte materials.
The resistive switching memory is typically a sandwich structure based on an easily oxidizable metal/solid electrolyte/inert metal, capable of forming a class of important non-volatile resistive switching memories (RRAM, resistive switching memory), commonly referred to as solid electrolyte base RRAM, programmable metallization cell (PMC), or a conductive bridge random access memory (CBRAM). This kind of memories has the advantages of simple structure, fast speed and low power consumption, and is regarded by the industry as one of the strong competitors of the next generation nonvolatile storage technology.
Its working principle is that, under the actuation of applied electric field, the easily anodic oxidizable metal of the metallic upper electrode A (Such as Cu, Ag and Ni, etc.) is oxidized to metal ions A+ under the action of electric field, metal ions A+ is transferred in the solid electrolyte B under the action of electric field, moving toward the cathode and finally reaching the inert lower electrode C, whereat reduced to metal A. As the metal is continuously deposited at the lower electrode C, finally reaching to the upper electrode A, thereby a plurality of filamentous metal conductive bridges connecting the upper and lower electrodes are formed, therefore the device resistance is in a low resistance state; under the action of the reverse electric field, the metal conductive bridges are disconnected, the device is restored to a high resistance state. These two resistive states can be converted to each other by the action of the applied electric field.
However, due to the commonly used inert metal electrode materials (such as Pt, Au, Pd and W, etc.) are polycrystalline structure, resulting that the metal atoms/ions are easily diffused into the inert electrode material to form alloy structure comprised of the easily oxidized metal and inert metal (literature 1, Ycyang, F. Pan, Q. Liu, M. Liu, and F. Zeng, Nano Lett. 9, 1636, 2009), the metal atoms/ions may also migrate to the surface of the inert electrode material through the inert electrode material (literature 2, J J Yang, J P Strachanm, Q. Xia, D A A Ohlberg, P J Kuekes, R D Kelley, W F Stickle, D R Stewart, G. Medeiros-Ribeiro, and R S Williams, Adv. Mater. 22, 4034, 2010). The diffusion of metal atoms/ions into the inert material is equivalent to the formation of easily oxidized metal source in the inert electrode, resulting in erroneous programming phenomenon (forming metal conductive filament under the reverse voltage) of such RRAM devices occurring in the reverse erase process (rupture process of conductive filament), negatively influencing the reliability of the device. At the same time, since the current limiting in this device erasing process is usually much greater than that in the programming process, the erroneous programming phenomenon during the erasing process can easily cause the device hard breakdown and the device failure, affecting the device reliability.